Discharge Coefficient in Siphon Spillway with Different Cross Sections

Transcription

1 World Applied Sciences Journal 17 (): , 01 ISSN IDOSI Publications, 01 Discharge Coefficient in Siphon Spillway with Different Cross Sections 1 Amin Ghafourian and Mohd. Nordin Adlan 1 School of Civil Engineering, Universiti Sains Malaysia, Nibong Tebal, Pulau pinang, Malaysia Department of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal Abstract: Siphon spillways are structures that use in many dams and irrigation networks and drainage. The siphon spillway works under increasing discharge like an ogee spillway. At a certain point of discharge, the priming occurs and the flow is pressurized for larger discharges. Estimating the relationship between discharge coefficient and discharge with reservoir level in this spillway is an important parameter in the designing process. Commonly, the cross sections of siphon spillways are rectangular. The main purpose of this research is to make a comparison between circular and rectangular cross section and investigate the changes of discharge coefficient in these cross sections. In order to achieve these purposes two section physical models of siphon spillway, with equal cross sections are made. In this study, all experiments were done in condition siphon spillway non aeration or spillway with black water. Key words:siphon spillway Physical model Discharge coefficient Circular cross section and rectangular cross section INTRODUCTION [3]. Based on the output water color, the siphons are divided in two categories: 1- siphon with black water Generally the spillways are installed to evacuate the (non aeration) - siphon with white water (aeration)[4]. excess water or over flow that the reservoir could not The first siphon spillway had poor design to evacuate store [1]. In diversion dams, the spillway is used to large quantities of water. The discharge coefficient of this bypass the over flow of the system. The spillway can be spillway was equal with 0.50 and it was gradually applied installed independently from the body of the dam, or it for greater amounts of water and the discharge coefficient can be part of it, similar to spillway in concrete dams [5]. was reached to []. Siphon spillways consist of the closed system with the The purpose of this research is to make two physical shape of inverted U. In installing this kind of spillways, models of siphon spillways having rectangular and the internal part of the upper curve of spillway is located circular cross sections and comparing discharge in normal level of dam. In the first process of increasing coefficient in these two cross sections and also with water level of the dam from normal level, excess water will constructed model by Gramatky (198)[] which had a be evacuated from the weir [6]. The act of siphon starts rectangular cross section and making a comparison when the air inside the upper bend located above the between acquired results from each one and determination crest is removed. Then due to the suction effect of gravity the best cross section under equal environmental force of water, underneath part of the siphon, the flow conditions (hydraulic conditions). In hydraulic and fluid would be consistent (Gramatky 198) []. science, the physical models have wide range of The main advantage of siphon spillway is that applications. with minor increase in the upper part of the water level, a In this research, determination of discharge significant increase is observed in the spillway discharge. coefficient the spillways having different cross sections The other advantages are their automatically work and with physical models was investigated, thus two physical their good performance without the need for models were designed with circular and rectangular cross mechanical instruments or other stimulants (Samani 1997) sections. Corresponding Author: Mohd. Nordin Adlan, Professor, Department of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal. 163

2 World Appl. Sci. J., 17 (): , 01 Fig. 1: Siphon with rectangular cross section Spillway Model with Circular Cross Section: The design of this model was based on the equality of cross sections in situations of both spillways. The diameter of the opening of this model is consistent along the length of the spillway as shown in Fig.. In this research due to little discharge and the need for exact reading of the height on the spillway to determine the output discharge from the structure was used a triangular spillway having the angel 90 degrees that is installed in the exit part of the flume. RESULTS AND DISCUSSION In this experiment, the discharge coefficients was compared in physical model of siphon spillway with rectangular and circular cross sections, under conditions that the spillway act under pressure, the spillway are positioned under the influence of eight different ranges of current, based on tank level that is shown in the Table1. These rates were selected based on tests performed Fig. : Siphon with circular cross section by Savage (001) and Grammatky (198) [, 10], in order to compare the results. Because in tests done by Savage the MATERIALS AND METHODS present conditions in siphon spillway with rectangular cross section, were similar to that of uncontrolled peak Siphon spillway is used in alongside or addition to spillway. The current rates were selected close to each other flood opening or intake of small power plants, such other to increase the accuracy of changes in discharge as described by Xian-Huan (1989) [7]. The hydraulic coefficient. design of a siphon mentions especially to the maximum In physical model for assuring about the precision of discharge [8]. In accordance with the energy equation the readings was repeated the tests five times with each rate flow rate Q is: of the current and if was observed a significant difference in the results omitted those readings and repeated the Q = AdV d= ab (gh 0) test. When the spillway channel is full of water and the siphon begins its action, in this situation the structure Where A d and V d are cross-section and average velocity acts under pressure. of the siphon outlet, a and b are height and width at the siphon crest and is a siphon efficiency The Effect of Discharge on Discharge Coefficient: coefficient [9]. For investigating the effect of discharge (Q) changing on discharge coefficient ( ) in siphon spillway having Design of Physical Model: The siphon spillways are rectangular and circular cross sections, the quantity of installed in a flume with 1000cm 60cm 50cm (L H W) in both cross sections of spillway would increase with size and a flow meter have been installed in the way increasing the discharge that is visible by comparing the to control and regulated the range of flow rate in increase of discharge in two cross sections, the increasing the flume. trend of discharge coefficient in circular cross section with respect to the rectangular one is higher as shown in Spillway Model with Rectangular Cross Section: Fig. 3. The required model made of Plexiglas with dimensions 48cm 60cm 50cm (L H W) which is shown in Fig. 1. Table 1: Water levels in tank The section has a lengthwise profile approximately proportional to peak spillway. Tank Level H1(m)

3 _ World Appl. Sci. J., 17 (): , The Effect of the Tank Level on Discharge Coefficient: H T is the tank level in upstream, in physical model of 0.85 siphon spillway with rectangular cross section, when 0.80 tank water level reaches to the height of 0.40 meter from the bottom of the fume, the current changes from 0.75 free one to under pressure one, of which this change in circular cross section happens in the height of meters. Comparing the coefficient discharge changes with 0.60 tank water level increase in two cross sections of siphon spillway shows that having a fixed and consistent Q (m 3/hr) level for tank water as is shown in Fig. 4, the spillway Circl Rectan with circular cross section has a higher discharge Fig. 3: The effect of Discharge (Q) on Discharge coefficient, in other words in a consistent level, the coefficient ( ) in two cross section circular cross section passes the more discharge than the circular one. Fig. 4: Comparing the coefficient discharge changes with increase H in two cross sections T The Effect of Internal Radius (ri) and the External Radius (ra) on Discharge Coefficient: ra and ri in the rectangular cross section model respectively equal 15 and 10 centimeters and in circular cross section they equal 0 and 7 centimeters, though in researches by Grammaty [] with rectangular cross section, ra and ri respectively are 30 and 4 cm. In order to investigate the effect of these two radiuses, the dimensionless parameter of ra/ri in all three models was achieved. For three ratios of ra/ri, the changes of discharge coefficient which is shown in Fig. 5. Q (m 3 /hr) Fig. 5: The effect of internal radius (ri) and the external radius (ra) on discharge coefficient Fig. 6: Comparing Reynolds number in two cross section The Reynolds Number (Rn) Changes: The increase of Rn is an indication for increased velocity, because the cross sections in models are consistent, in other words discharge and discharge coefficients are increased. The increase of Rn would result in a more turbulent flow. This turbulent in a small scale (the model of spillway) may consist of numerous small vortex currents that due to their viscosity convert the mechanical energy to non returnable. The mentioned cause and also the separation phenomenon or returnable current, especially after the deflector in rectangular cross section model, are among the items that increase with higher Rn and decrease the pressure. With comparing the changes of discharge coefficient in rectangular and circular cross sections of siphon spillways based on Reynolds number as shown in Fig. 6 is observed that in a consistent level of tank water in both cross sections of spillway, the flow in circular cross section has a higher Reynolds number, that is because of higher velocity in this cross section than rectangular one. 165

4 4 P a g H T ra f Rn, Fr,, l = a ri qb World Appl. Sci. J., 17 (): , 01 RESULTS AND DISCUSSION mentioned diagrams, the circular cross section has a higher discharge coefficient and with increase in this ratio, Discharge coefficient in siphon spillway having then discharge increases too. It means that negative rectangular cross section is in the range of 0.73 (for critical pressures occur in more discharges, so the circular discharge of 3 cubic meter per hour) up to 0.80 (for cross section having more ratio of (ra/ri) has priority over discharge of 6 cubic meter per hour) and for circular those two cross sections. cross section in the range of 0.71 (for discharge of 1 With increase of rate head of spillway is suggested to cubic meter per hour) up to 0.86 (for discharge of 8 cubic build the exit part conversely. This case is in accordance meter per hour). In researches done by Grammaty (198)[] with the researches of Hager and his colleagues (1991) for similar conditions, the coefficient between 0.7 till 0.81 [11]. was achieved, of which the high discharge coefficient in Based on statistical analysis and the correlation circular cross section over the rectangular one shows that between dependent variables and independent ones, the this one is better and more suitable. most important parameter from the point of effectiveness Comparing the increase of discharge in two cross in regression equation, is Reynolds number. sections of siphon spillway is observed that the increase The increase in Reynolds number in model would trend of discharge coefficient in circular cross section cause an increase in the coefficient of discharge across over the rectangular one is much more. the length of the structure. Hager and Grammaty's The transition from free flow to under pressure state researches confirm these results. Among the resulted in siphon spillway with circular spillway happens in a equations by regression, the equation of three lower level (36.5cm from the bottom of the flume) with q independent parameters Reynolds number and the respect to the rectangular cross section (40cm from gh 3 the bottom of t flume), which is an advantage for ratio of the radiuses have an acceptable precision, due to rapid evacuation of the tank in the vent of flood a higher regression (0.98) over other equations. occurrence. With comparing the changes of discharge coefficient with tank water increase in two cross sections of siphon CONCLUSION spillway is observed that with a consistent level for tank In the present experimental work, by using two water, the spillway with circular cross section has a higher physical models of siphon spillway with different cross discharge coefficient, in other words in a consistent level, section, discharge coefficient in two sections was studied. the circular cross section passes more discharge than the Experiments show that discharge coefficient in siphon rectangular one. spillway with circular cross section is higher than With comparing the changes of discharge rectangular section that this one is better and more coefficients in circular and rectangular cross sections of suitable for rapid depletion of water from reservoir. siphon spillway, based on Reynolds number is resulted that in a consistent level for tank water in both cross sections, the flow in circular one has a higher Reynolds List of Notation number. = Density; According to the Buckingham theorem, the µ = Dynamic viscosity; important and effective parameters on the flow are as g = Gravity acceleration; follows: V = Velocity; The ratio of (ra/ri) in the model having rectangular cross section equals 1.5, in circular one is.85 and in the researches of Grammaty [] it equals 1.7. According to HT = Total head or the difference between tank water level and downstream; = Pressure changes across the spillway; p L a b d ri ra = The height of rectangular cross section of siphon; = The width of rectangular cross section of siphon; = The diameter of circular cross section; = Internal radius; and = External radius. 166

5 World Appl. Sci. J., 17 (): , 01 REFERENCES 7. Featherstone, R.E. and C. Nalluri, Civil Engineering Hydraulics, Blackwell Science. 1. Davies, B., M. Larson and G. Wright, 004, Fool 8. Dornack, S. and H.B. Horlaacher, Hydraulic Hollow Spillway Rehabilitation Project, Journal of ModelExperiments for the siphon spillway of the ASCE. Oker Dam. 8th IAHR Congress, Graz, Osterreich,. Gramathy, G. and K. Robinson, Siphon pp: Spillway, Thesis, California Institute of Technology 9. Head, C.R., Low-head air-regulated siphons, J. 3. M.Samani, Hydraulic Structures Design, Hydraulics Division, ASCE. 101(HY3), pp: Consulting Engineers Dzab 10. Savage, Frizell, Crowder, 003. Brains Versus Brawn: 4. Bollrich, G., Hydraulic investigations of the The Changing World of Hydraulic Model Studies, high-head siphon spillway of Burgkhammer. 18th Mc Neese State University. ICOLD Congress Durban Q71 (R), pp: Aigner, D. and H.B. Horlacher, 007. Investigation of 5. Vischer, D.L. and W.H. Hager, DAM aerated siphon. 3th IAHR, 1: 31. HYDRAULICS, John Wiley & Sons, pp: Lee, W. and J.A. Hoopes, Prediction of Cavitation Damage for Spillways, Journal of Hydr. Engrg., VOL. 1, Sep. 167